a, Size-exclusion chromatography of NOMPC after exchange from DDM into amphipol A8-35. The peak fraction corresponding to NOMPC tetramer (indicated by arrow) was examined by SDS–polyacrylamide gel electrophoresis. The NOMPC monomer (∼190 kD) band is indicated by arrow head. The upper bands may correspond to incompletely disassociated NOMPC oligomers. b, Size-exclusion chromatography of NOMPC reconstituted into lipid nanodisc with MSP2N2. The peak fraction corresponding to NOMPC is indicated by arrow. c, d, Raw micrographs of NOMPC-amphipol (c) and NOMPC–nanodisc (d) samples examined by negative stain EM. Both showed mono dispersed and homogeneous particles. e, f, 2D class averages of NOMPC particles in amphipol (e) or nanodisc (f) by negative stain EM, with the nanodisc sample showing better ordered features than the amphipol sample.